Solutions for the treatment of amorphous carbon or graphite manufactured articles for improving their resistance to oxidation

ABSTRACT

THIS INVENTION PROVIDED A SOLUTION AND NOVEL PROCEDURE FOR IMPARTING OXIDATION RESISTANCE TO AMORPHOUS CARBON OR GRAPHITE MANUFACTURED ARTICLES BY MEANS OF THE COATING SOLUTION WHICH CONSISTS OF WATER, PHOSPHORIC ACID, ONE OR MORE COMPOUNDS SELECTED FROM THE GROUPS OF SALTS, OXIDES AND HYDROXIDES OF A METAL OF THE 2ND GROUP OF THE PERIODIC SYSTEM; ONE OR BOTH COMPONENTS OF THE GROUP FORMED BY BORIC AND SOLDIUM TETRABORATE DECAHYDRATE AND ONE OR MORE COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF NITRIC ACID, SODIUM NITRITE AND SODIUM CARBONATE.

J 1974 1 F.-BALDIERI ETAL 3,814,699

SOLUTIONS FOR THE ,TREA'X'IBN'I' OF AMORPHOUS CARBON OR GRAPHITE "ARUPACTURED ARTICLES FOR IMPROVING THEIR RESISTANCHTQOKIDATION Filed Jan. 22. .1971

properties, used in many 19,6611?! t v to. crown 3/00.- non urn-m 1/20 us, (.1. 252-391 V 1 Claims ABSTRACT OF THE DISCLOSURE This invention provides a solution and novel procedure for imparting oxidation resistance to amorphous carbon or graphite manufactured articlesby means of the coatin; solution which consists of waten phosphoric acid, one -ormore compounds selected from the groups of salts, oxides and hydroxides of a metal of the Znd-group of the periodic system; one or both components of the group formed by boric and sodium tetrahorate deeahydrate and one or more compounds, selected from the group consisting of nitric acid, sodium nitrite and sodium carbonate.

The present invention refers to a process designed to give amorphous carbon or graphite manufactured articles greater resistance to oxidationat high temperature; reference is made, in addition, moreparticularly to'a series of solutions designed to make the said'manufacmred articles more resistant to oxidation at high temperature, and to the said manufactured articles which are obtained by such processes and solutions. It is known that cariion materials-whether amorphous or crystalline-are, in view of their particular electric, mechanical and thermal types ofv industries as electrodes for furnaces in the iron and steel in us ry. or, again as electrodes, in the production of aluminum, or asrefrac- Despite their wide field of use, carbon materials have always found a limit in their easy oxidability at high tem- Proposals have been made for coverings protecting the iacleets on the surface of the manufactured article, with a have of silicone, polysiloxnns, colloidal silica, carbides, nitrides and silicidea of refractory metals, etc; these systerm are not the'best, both because of the complexity of the operations and equipments required for the coating, and the 1110mm layer come; away fairly easily, and also owing to the addition of elements which are sometimes which my happen when menufaanred articlen'protected in this way are used as elec-' tmdee or retrectoriesin-the iron and steel industry orin metallurgy in general. V

Trials have also been made with impregnation with compound; as ammonium 1: plane, men-aim or zinc of alkaline metals, sodium tetrahorate, etc. Bot-alto these methods' do not ennwe good protection against oxidation except at relatively low teunmatmes.

fine at the present invention wooden and relations capable 01' making in: manctactnred articles resistant to the in air, molten motels; or other high carbon or mph oxidation canned e oxidation present'lnvention will be illustrated in the description given hereunder.

The obieetivesof the present invention are achieved by impregnating, orv covering in some other way, the caution or graphitermanufactured articles, using a solutionimmnoted of:

. water phosphoric acid 1 one or more compounds selected in one or more" of: the

groups formed by the salts, oxides, a metal of the 2nd group ofthe periodic system one or both the'components of the group formed by-borie acid and sodium tetrabora'te decahy'drate 1 one or more compounds selected from among nitric acid,

sodium nitrite, and sodium carbonate.

The advantages of using such a composition with regard to protection from oxidation will be evident in the examistosupplyn means, in a wider temperature ram tlnintliat hitherto covered. Another mom. equally important,

supply carbon or graphite mnnlactmed articles cnpal'tle o! resisting high temperature oxidation better titan those so in: town. The ways tosciiieve the dome ohjeetivw and the advantages deriving from the of this inpies which follow. Other immediate advantages of these solutions consist in not using hydrochloric acid as solvent and in providing for relatively low concentrations of pilot 1 phoric acid, thus making the preparation, handling and application of the solutions easier; fllrthermore the process selected for the preparation of the manufactured articlesdoes not require their baking, so that it is simpler and more economical quire it.

According to the general idea of the present invention, the solutions inht g oxidation may have a percentage :otnposition by weight varying within the ranges indicated eiow:

Percent Water 57.0469 One or more compounds selected from among the salts, oxides and hydroxides of a metal of 2nd group 0.14.0 One or both the components of the group formed by sodium tctraborate decahydrate and horic acid .l6.92.0 Phosphoric acid 26.0-40.0 One or more compounds seloctedfrom among nitric acid, sodium nitrite, and sodium A study of such solutions has shown that they can he fled into two groups, zoom-dim towbeiher sodium tetraborate decahydrate or boric is used.

in the two cases the of the solntons can vary within the following limitn'again expreeaied as a percoinage compoa'tion by weight: 1 I Percent Water 57.0-67.0 Phosphoric acid 20.0463 One or more selected from among 'tbemltsmxidesnndhydrofametalof I the 'Zntl. group 5.0414 Sodium tetrahorate decahydrste 12.0-16.6. One or more compmmdsa'eleeted from-among nitric acid, sodium nitrite, and sodium car- We, Dune 6,M-9

Water mower) Phosphoric acid -t...-.. .W lib-i615 Ono or more compounds 'selected from among the s ite, oxides, and hydrosirles'ot a metal v ul' the 2nd group caution, 2.0-0.8 Boric acid .--.M.,.;...' lil -7 0 honttte SAX (Ml and hydroxides of than other processes, which do 1'6? 11L: .i i s i a If lorthinthefotlowingeamfles. w Page; W1 I v Phosphoricaeid 1, tale a arn-put weighing 0.450 ew cm M ocarboniflm 'amElettt j, ,brand'electrode'diameter 350 mm. was placed lit-an emptiable container and subiectedto degassing with a vacuum up to 5.10"" mm. Hg; once sample had been degamed, the impregnating solution was introduced into the container, still under vacuum,- at a temperature of 70 C., so that it covered the sample-,the

pressure isr'aised toa maximum of s-atmospheres. The Pie. removed from the. solution, "was dried and psi seasoned at room pressure. and temperature for-about 48 hours. t

The impregnating solution had the following percentage compositionhy-weightz; t E

.Percent Water 62.8 Phosphoric acid v 18.0 Calcium'oxide a 0.4 Sodium tetrab'orate' decahydrate- 14.6' I Nitric acid 2.9 Sodium nitrate 0.3 Sodium carbonate 1.0 v

To prepare the solution the phosphoric acid first added to the water, then the calcium' oxide was added, and then the sodium tetraboratc dccahydratm'until saturation at 70 C., and lastly all the other components.

The sample thus treated wassubie'cted to oxidation rcristance testingby means of a thermobalance, with air flow equal 10.6.4 liters/h. and heating speed of 10 'C./ rnin., up to a maximum temperature of 1600 C. In this test the percentage decrease in the total weight of the sample .was measured. The results were as follows:

' Total percentage Temperature: decrease in weight Up to IOOO C. 8.2 Up to 1250 C 22.8 Up to 1400 C 39.2 Up to 1600 C 63.8

EXAMPLE 2 A sample having the same characteristics as the one in example 1 and obtained from" the same zone of the electrodewas treated, on the basis of the technique described above, with a solution having the following composition The solution was prepared in the way described in cit ample l. 'Ihe thermobalance test, carried out with the same procedure as in example gave the following results:

Total percentage A sample having the same characteristics as the one in p 1 Total percentage de Temperature: crease inweight I -Up to1000 C. j

Up to' 1250' C. 20.4

Y Temperature:

thermobalance steamer was up, oedure as in example 1,.gavethe followingresultst I :Totalperccntagedeis A M has;

in example 1 and obtained tromthe same zone of the electrode was treated; on the basis of the technique, described above, with a solution having the. following thcrmobalance test, carried out with the same pro I ccdure'as' in example l, gave the following results:

A sampleisample D) of the already. used was treated, 'on the basis or the technique described above,

with a solution having the following percentage composition by weight:

v iercent Water. 74.2

, Phosphoric acid 20.4 Calcium oxide 0.4

Boric acid 5.0

By oif the thermobalance test, carried out with thesameprooedure as in example i, the following were. I

dctcrmincdz. 1

- (a) Temperature of start of reaction (b) Speedoi reaction expressed as percentage reduction in weight of the sample per 7C. (percent C.--) (cl'The total percentage decrease in weight oi the sample upto the temperature t C.

by weight: g

a y Percent Water r i 62.8 Phosphoric acid 18.0 Zinc oxide 0.4 Sodium tetraborate decahydratc 14.6

Nitric acid 2.9' Sodium nitrite v 0.3 Sodium carbonate -s 1.0

results of the test are shown in tables 1 and 2. v

. EXAMPLE 6 v v A sample (sample C) of type described above was treated, onthe basis oi 'the technique described above,

' with a solution having the following percentage composi- Percent Water 64.8

Phosphoric acid 18.6

Calcium oxide 0.4

Sodium tctrahorat'e decahydratc u--. l$.l

Sodium carbonate s. Ll

example 1 and obtained from theeame zone 02 the clectrade was treated, on the basis of the technique described "tion' by weight:

By means 0! the thcrmohalancetest, the same determinutiona as in sample 5 were made. The results'obtained are shown in tables l and 2.

In order to emphasize the advantages of the series or solutions covered by the present invention, a comparison" crease in weight P P w16o0 c. 7m

A sample havingthe same characteristics one a solution and the method Exampl 510. 5.

Percent Water a 21.9 Phosphoric acid 63.3 Zinc oxide 13.8 Boric acid 1.0

The impregnation vessel containing the sample was de gassed and then filled, still under vacuum, with the above solution. impregnation under pressure was then carried out. with a maximum pressure of 5 atmospheres.

After impregnation the sample was dried and then subjected to'desiccation at 105' C. and baking at 500 C.

'The tests were then carried out with the thermobalance in accordance with example 5 and with the same proce dure as in example 1. The results obtained are shown in tables 1 and 2.

. 'rannnt Sample t A. B C D. M

'Iemperntureolstortotreactlon.C. 5w 620 620 Percent wt. decrease (percent Cr):

' GZOISIIPC. 0.015 0.009 0.010 0. 0.100. 0.038 0.003 0. J4; 0.013 0.054 0. 0.150 0.107 0.106. 0. 0.135 0137 0. TABLEZ Percent ot Sampls.. A B C v D In order better to evidence the properties of the solutions covered by the present invention,-,other experiments were carried out, the results of which are shown here under.

by the samples of 4 hours at the temperature r.

The tests were made in a mufi'ie withlan air circulation obtained by natural draught and the samples, about 160 g.. were placed in a had reached the working temperature.

The percentage increase in weight of the samples, due

to impregnation, was always at least 5% after seasoning.

Tables 4 and 5 show the results obtained on the thermobalance when using the solution of example 5. but varying once the calcium oxide and once the phosphoric acid respectively. and maintaining the other components constantin order to demonstrate the importance of the composition of the solutions for purposes. of good protection;

it is found that the percentages used in the solution of example 5 are the most suitable ones.

TABLE 3 renczrr'raon Loss AFTER 4 nouns AT '1" as Pmza'runn no. 7 E

r c Non-treat d mpltlto Example No. 2. Example No. 3.

Exampl No. 0

essa s s magnesite crucible when the muttle The attached graph shows the results ,of some experiments which give a comparison between non-treated graphite (curve a); graphite treated with ,the solution already kno'wn to technique, and used in description after example'6 (curve b); graphite treated according to example 5 (curve c);' graphite treated according to example 6 (curve d). The ordinates of the graph represent thepercentage losses in weight of the sample when heated up to temperature r C., as shown in the abscissaetwith heatpresent invention.

ing speed of 10 C./min. in air flow equal to 6.4liters/h. From the foregoing examples and comparisons it can be seen that there is a clear improvement in the properties of resistance to oxidation that the'solutions proposed as new by the present invention are capable of giving to graphite or carbon bodies, as compared with the results which can be obtained with solutions already known in technique. .Itshould be borne: in mind that, although the" samples tested in'the various experiments were all obtained from electrodes, the present invention refers to the treatment of any type of manufacturedjarticles made from amorphous carbon or graphite, such for example as crucibles. refractory bricks, resistors and alsotnaterials treated with carbon products. Furthermore, although the examples have mentionedonly one particular technique of impregnation, it is clear that the present invention also refers to any other technique of application (varnishing,.lim mersion, etc.) of the solutions forming the subject of the What we claim is:

1. Solution suitable for treatment of manufactured arti- I graphite, for-the purcles made ofarnorphous carbon or pose of improving their resistance to oxidationgcharacterized by the fact of being formed essentially of: water: phosphoric acid; one or more compounds selected from i the oxides and hydroxides of a metal of the 2nd group of Table 3 shows the percent losses in weightaiter a stay j phosphoric acid; one or more the periodic system; one or both the components of the ,"group formed by boric acid and sodium tetraborate decahydrate; oneor more compounds selected from the group formed by nitric acid; sodium nitrite, and sodium carbon- .ate';' said constituents of the solution being mixed in accordance with percentage ratiosby weight ranging be tween 57.0:26.0:0.l:l6.9:0.0 and rspectively;

2. Solutionin accordance; with claim 1 in which" the" group of the Periodic System is calcium.

metal of the 2nd 3. Solution in metal of the 2nd accordance with claim 1 in which the group of the. Periodic System is cadmium.

4. Solution suitable for treatment of manufactured articlesrnade of amorphous carbon or graphite, for the purpose of improving their resistance to oxidation, characterized by the fact of being formed essentially of: water; components selected from '1 the oxides and hydroxides'of a metal of the 2nd group of the periodic system;- or more compounds selected from the group formed by nitric acid, sodium nitrite.

sodium carbonate; said constituentn oithesolution being mixed in accordance with percentage ratios by weight ranging between 57.0:200: 5.0: l2.0:6.0 and 67.0:l6.0:0.4:i6.6:0.0, respectively.

of manufactured graphite. for the 5. Solution suitable for treatment articles made of amorphous carbon or ium tetraborate dccahydrate; one

Periodic System; boric acid: one or more compound: selected from the group formed by nitric acid, sodium ortrite, sodium carbonate; e'nid constituents of the solution being miudvin accordnnce with percentage ratio: by weight ranging between .70.0:2 1.0:2.0:2.0:$.0 and 76:0 16.6:0.4:7.0 :0.0, respectively.

6. Solution suitable for treatment of manufactured articles made amorphous carbon or graphite which con aisle essentially of water, phosphoric acid, calcium oxide and boric acid in eccordancewith percentage ratio: by weight ranging between 57.0:26.0:0.l 16.9 and 76.0: 10.0: 6.0:2.0,- respectively.

7. Solution according to claim 6 which includes up to 6.0% of one or more compound: selected from the group consisting of nitric acid, sodium nitrite and sodium car-v bonate.

3/1950 Gibson 148-6." 2 12/1926 Coelett 148-6152 12/1960 8/1893 Garleis et al. 117-228 1/1971 Rubisch 117-169 R 11/1966 Harwelletal. -.L 1 48-6.15

r 7-.. 148-615 rt Colemnn..-...---- 148-615- R 

